Intergranular Attack
Description
Metals and alloys are composed of grains similar to sand
grains in a common sandstone. Intergranular corrosion refers to
the selective corrosion of the grain boundary regions. This
attack is very common in some stainless steels and nickel alloys.
Some aluminum alloys can also exhibit intergranular and
exfoliation (i.e. corrosion at grain boundary sites parallel to
the metal surface where corrosion products force apart the
metal).
Prevention or Remedial Action
- Heat treatment of alloy to remove phases from grain
boundary regions which reduce corrosion resistance (i.e.
solution annealing).
- use modified alloys which have eliminated such grain
boundary phases through stabilizing elements or reduced
levels of impurities:
EXAMPLE: stainless steels such as AISI 304 or 316 can be
"sensitized" by heating or welding in the range
900 to 1500 F. This forms carbide precipitates which
reduce corrosion resistance of grain boundaries. The use
of low carbon 304L or 316L will increase resistance to
inter granular corrosion in welded components. for
prolonged service at high temperature stabilized
stainless steels (i.e. aisi 321 and 347) will increase
resistance to inter granular corrosion.
Standard Test Methods
- ASTM A 262 - practices for detecting susceptibility to
intergranular attack in austenitic stainless steels.
- ASTM G-28 - test methods for detecting susceptibility to
intergranular attack in wrought, nickel rich,
chromium-bearing alloys.
- ASTM G-34 - test method for exfoliation corrosion
susceptibility in 2xxx and 7xxx series aluminum alloys
(EXCO test).
- ASTM G-66 - test method for visual assessment of
exfoliation corrosion susceptibility of 5xxx series
aluminum alloys (asset test).
- ASTM G-67 - test method for determining the
susceptibility to inter granular corrosion of 5xxx series
aluminum alloys by mass loss after exposure to nitric
acid (namlt test).
Evaluation of Intergranular Attack
The most common concern for stainless alloys is the influence
of welding and/or heat treatment on susceptibility to
intergranular corrosion produced by carbide precipitation (i.e.
senitization). Therefore, the carbon content is an important
metallurgical consideration with lower carbon (and nitrogen)
materials or materials that have been stabilized with additions
of Ti or Nb showing lower tendencies to intergranular corrosion.
In evaluation, the tendencies for intergranular corrosion can
vary greatly depending on the severity of the test conditions and
environment. Oftentimes, standardized environments are used such
as those given in ASTM A262.
Intergranular corrosion various alloys requires the use of
different environments:
- Aluminum alloys - acidified NaCl/HCl solution or HNO3solution.
- Magnesium alloys - NaCl/HF solution
- Copper alloys - NaCl solution with H2SO4
or HNO3.
- Lead alloys - Acetic acid or HF solutions
